Asthma is amongst the commonest chronic illnesses of childhood yet there is no single objective test that results in a definitive diagnosis. Diagnosis remains clinical, especially in young children. At present, asthma diagnosis in children is best viewed as a process. The initial steps include a clinical assessment focusing on recognising characteristic clinical features from a detailed history and examination combined with a careful review of possible alternative diagnoses. Since there is no gold standard for diagnosis, clinical evaluation can only give a probability that a given child has a diagnosis of asthma. If the probability of asthma is high, the next step is frequently a trial of anti-asthma treatment, usually a low-dose of an inhaled corticosteroid and a bronchodilator. Fairly to respond to a properly taken anti-asthma treatment suggests an alternative diagnosis. If the probability is low, a diagnosis other than asthma may be suspected. Then further investigation and specialist referral may be indicated. In those with an intermediate probability tests of lung function (expiratory airway obstruction, bronchial reversibility and occasionally bronchial hyperreactivity), and tests for atopy may be helpful.